1. Field of the Invention
The present invention relates generally to the field of fuel cells and, more specifically, to an arrangement for rapidly increasing power output from a direct oxidation fuel cell through the introduction of neat or concentrated fuel to the cell""s anode flow field channel, anode diffusion layer or protonically-conductive membrane.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive choices for fuel due to the their high specific energy.
Fuel systems may be divided into xe2x80x9creformer-basedxe2x80x9d (i.e., those in which the fuel is processed in some fashion before it is introduced into the cell) or xe2x80x9cdirect oxidationxe2x80x9d in which the fuel is fed directly into the cell without internal processing. Most currently available fuel cells are of the reformer-based type, but fuel-processing requirements for such cells limits the applicability of those cells to relatively large systems.
Direct oxidation fuel cell systems may be better suited for a number of applications such as smaller mobile devices (i.e., mobile phones, handheld and laptop computers), as well as in larger applications. One example of a direct oxidation system is the direct methanol fuel cell system or DMFC. In a DMFC, the electrochemical reaction at the anode is a conversion of methanol and water to CO2, H+ and exe2x88x92. More specifically, a liquid carbonaceous fuel solution (typically an aqueous methanol solution) is applied to a protonically-conductive (but electronically non-conductive) membrane (PCM) directly using a catalyst on the membrane surface to enable direct oxidation of the hydrocarbon on the anode. The hydrogen protons are separated from the electrons and the protons pass through the PCM, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and travel through a load, providing electrical power.
The reactions that generate electrical power occur nearly instantaneously when fuel is applied to the PCM, as long as the load draws electricity from the fuel cell. However, presently available PCMs are permeable to methanol, allowing methanol to pass from the anode face of the PCM, to the cathode face of the PCM, where the methanol is oxidized upon contact with air without generating electricity. This phenomenon is known as xe2x80x9cmethanol crossover.xe2x80x9d However, by introducing methanol in an aqueous solution, methanol crossover is minimized. This water and methanol mixture provides a sufficient amount of methanol to provide power to the load under low power demand conditions, while the water prevents excess methanol crossover from occurring. However, if power demand is high, or increases rapidly, there may be insufficient methanol in the aqueous methanol solution to provide sufficient power to the load in an acceptable time period.
The carbon dioxide, which is essentially a waste product, is separated from the remaining methanol fuel mixture before such fuel is re-circulated. In an alternative usage of the carbon dioxide this gas can be used to passively pump liquid methanol into the feed fuel cell. This is disclosed in U.S. patent application Ser. No. 09/717,754, filed on Dec. 8, 2000, for a PASSIVELY PUMPED LIQUID FEED FUEL CELL SYSTEM, which is commonly owned by the assignee of the present invention, and which is incorporated by reference herein in its entirety.
Substantial research has been dedicated to development of DMFC systems for use in portable electronics in recent years. However, present DMFC system designs are not able to meet the power demand profiles required for portable electronics while satisfying the desired form factors. In other words, the electrical power demand of current generation portable electronic devices (as well as the power needs of future portable electronics) may change very quickly, depending on their operating state, whereas present DMFC designs are not able to increase or decrease their power output rapidly, due to the fact that sufficient fuel is not available from the dilute fuel mixture. While it is possible to increase or decrease the concentration of methanol in the fuel mixture within the pump (or other mixing apparatus) being applied to the membrane electrode assembly (MEA) of a DMFC in response to power demands, there is an unacceptable delay between the time that power is demanded and the response of the DMFC. The delay is primarily attributable to the time that it takes to add more concentrated fuel from a fuel source to the fuel mixture, the time that it takes for the more concentrated fuel to be disseminated into the fuel mixture within the pump, and the time required to transport additional fuel through the system and to the PCM through the diffusion layer.
It is common practice to increase the voltage level of a DMFC system by connecting several fuel cells together as a series electrical circuit. One method of creating such an electrical connection is to fabricate a xe2x80x9cstackxe2x80x9d of DMFCs whereby a bipolar plate is placed in physical contact with the cathode flow field plate of a first cell and the anode flow field plate of a second cell. By doing so, the voltage output of the DMFC system is in creased arithmetically, thus providing increased power output to meet the demands of a power application.
In order to provide a more rapid response time, some current designs envision electrically coupling the DMFC with a capacitor or battery to meet the instantaneous power demands of a given application. However, such an approach requires additional components and connection, making control of the system more complicated, and presents increased difficulty in manufacturing on a commercial scale. In addition, each of these components has performance shortcomings based on the limitations of the technologies including, but not limited to recharge time and life cycle of each.
In brief summary, the present invention provides a direct oxidation fuel cell system in which the output power level may be increased rapidly to meet the power demands of a desired application, including typical portable electronic devices. In accordance with a preferred embodiment of the invention, an alternative fuel flow path is provided between a fuel source and an anode flow field plate. By actuating one or more valves, neat or concentrated fuel may be introduced into the flow field channels from which it diffuses through the diffusion layer and is introduced to the PCM. Because the alternative fuel flow path effectively bypasses the normal fuel flow path, the time delay until power output increases is greatly reduced.
In accordance with a second embodiment of the invention, an alternative fuel flow path is provided between a fuel source and the anode diffusion layer of the fuel cell. Again, by actuating one or more valves, neat or concentrated fuel may be introduced into the diffusion layer, decreasing the time delay between the demand for power and the increased generation of power. The time delay may be further shortened by delivering the concentrated fuel to the diffusion layer via a pump or pressurized vessel, thus providing a greater diffusion rate within the diffusion layer.
In accordance with a third embodiment of the invention, an alternative fuel flow path is provided from the fuel source to the PCM. Because the alternative fuel flow path effectively bypasses the normal fuel flow path and anode diffusion layer, the time delay until power output increases is greatly reduced.
The present invention may minimize or eliminate the need for batteries or capacitors, thereby simplifying the overall system structure, improving manufacturability, and reducing cost, volume and weight.